The invention relates to a harmonic reduction gear and to a safety apparatus for motor vehicles comprising a reduction gear of this kind.
Harmonic reduction gears, which are also called cycloidal gears, have the advantage that rotational motions can be stepped down to an extreme extent. For this the outer toothed ring of an outer toothed member which is driven by the eccenter, in general cooperates with a surrounding inner toothed ring, which is fixed to the housing, in such a manner that the outer toothed member rolls on the inner toothed ring in the rotational sense which is opposite to that of the eccenter. For this the outer toothed member has a diameter which is so much smaller than that of the surrounding inner toothed ring that a radially one-sided rolling motion is possible. Through the fact that the outer toothed member has fewer teeth than the surrounded inner toothed ring, the reduction ratio is determined in dependence on the ratio of the numbers of teeth of the outer toothed member and the inner toothed ring.
Reduction gears of this kind are used to particular advantage in belt winders of safety belt apparatuses in motor vehicles (see e.g. U.S. Pat. No. 5 518 197 and EP 0 844 152 A) in order to actuate a draw-out blocking, when the safety belt is largely drawn out or largely wound up, respectively, in such a manner that it blocks or again enables the drawing out of the belt, respectively. Through this it can be prevented in particular when strapping in children that the latter draw out the safety belt by playing with the safety belt holding them to such an extent that the safety retention function is impaired. Since the belt winder makes a large number of rotations when being drawn from the fully drawn in to the fully drawn out position, it is a matter of placing a reduction gear between the draw-out blocking and the belt winder which steps down the rotary movement to such an extent that a defined switching operation is triggered only at each end of the belt draw-out movement.
Disadvantageous in the known harmonic reduction gears is that two components are required which are provided with teeth being arranged all around. Gear wheels or toothed rings, respectively, are however rather complicated and expensive to manufacture and also contribute to an undesirable weight increase in particular in the safety belt apparatuses for motor vehicles.
The object of the invention is thus to provide a reduction gear of the initially named kind in particular for safety belt apparatuses in motor vehicles having a switchable draw-out blocking, which has a considerably simplified construction so that in particular it can be manufactured more economically, and which also enables a weight reduction.
The idea of the invention is thus to be seen in that the outer toothed member, which is provided with an outer toothed ring which extends over the entire periphery, is reduced to a rotary arrangement having only two diametrically opposed outer teeth left. The idea of the invention is also not departed from when instead of a single tooth on each side of the axis of rotation a few, in particular two and in the extreme case three, teeth are provided.
In this way the diametrically opposed teeth are alternatingly brought into and out of engagement with the inner toothed ring, with in each case a peripheral displacement of the teeth opposite to the rotational sense of the eccenter by preferably one tooth in each case taking place through the eccenter.
Thus, as a result of the design in accordance with the invention all the teeth of the previously usual outer toothed ring with the exception of preferably two are omitted, which in addition to a highly simplified manufacture also brings about a considerable saving in weight.
The two teeth are preferably arranged on diametrically opposed arms.
In order to perfectly control the pivotal movement occurring during the passage from one tooth depression into the next one, of the respective tooth which has left a tooth depression, the support surfaces are expedient, which ensure a perfect conduction of a tooth into each next tooth depression. These support surfaces are preferably arranged on diametrically opposed arms too.
The arms of the teeth and of the support surfaces preferably form a cross arrangement, the beams of which are mutually displaced by 90xc2x0 in each case.
The opening which is provided for the eccenter is located radially inwardly in the arms or in the cross arrangement, respectively, and indeed in accordance with claim 7 in each case in the middle.
The abutment means for switching apparatus are to be attached in a suitable manner to the outer toothed member having only two teeth.
The safety belt apparatus in accordance with the invention the eccenter is rotationally fixedly coupled to the belt winder, while the abutment means are arranged on the outer toothed member having only two teeth.